(1) Field of the Invention
The present invention relates to an easily-openable heat-seal lid. More particularly, the present invention relates to a heat-seal lid of the metal foil type having a good easy openability and an excellent sealing property in combination.
(2) Description of the Prior Art
A so-called peelable seal lid is known as a heat-seal lid having both the sealing property and easy openability. This peelable seal lid comprises a flexible substrate such as a metal foil and a layer of a heat sealant formed thereon. A heat sealant which is formed by incorporating a wax, a tackifier and an elastomer into an olefin type resin such as polyethylene so that the seal strength is controlled to an order of 1 Kg/15 mm is used. A peelable seal lid of this type, however, is poor in the sealing reliability in the case where a packaging vessel filled with a content and sealed by this lid is subjected to heat sterilization, and according to Official Notice No. 17 of the Welfare Ministry (enforced on Aug. 1, 1977), it is stipulated that a food vessel which is sealed by heat sealing and sterilized by heating under pressure should have a heat seal strength of at least 2.3 Kg/15 mm.
A heat-seal lid sealed to the vessel with such a high seal strength cannot easily be peeled on the heat-sealed interface by hand, and at the present, opening is ordinarily performed by using a tool such as a knife or a can opener.
A so-called easy-open can lid is known as the lid that can easily be peeled by hand without using any particular tool. This can lid is formed by defining an opening on a can lid formed of an aluminum sheet by a score, forming a rivet on the opening integrally with the can lid and securing a pull ring through this rivet on the opening, and this can lid is double-seamed to a flange of a can body.
Thuis easy-open can lid can resist sterilization by heating under pressure and has an excellent openability, but since an aluminum material is used in a large quantity and the can lid is prepared through complicated, troublesome and severe forming steps, the price of the easy-open can lid is considerably higher than that of the above-mentioned flexible heat-seal lid.
Furthermore, this easy-open can is defective in that a vessel to which the easy-open can lid can be applied is limited to a vessel formed of a material having a high heat resistance. More specifically, when this easy-open can is double-seamed to a plastic cup-shaped vessel prepared by draw forming, it is difficult to obtain sealing having a high reliability, and in this case, under hot filling or retort sterilization conditions, safe sealing becomes more difficult because the flange of the cup-shaped vessel is softened at the high temperature adopted.
As another easily-openable heat-seal lid, there is known a lid prepared by forming an opening in a lid member composed of a laminate including a metal foil, a paper substrate and a plastic film and forming an opening tab of a plastic material integrally with the lid member by injection molding so that the tab covers the opening and adheres closely to the peripheral portion of the opening. The heat-seal lid of this type is defective in that it is very difficult to simultaneously attain a good adhering and sealing property of the tab to the peripheral portion of the opening and a good easy openability of the tab.